This invention relates to leadframe-based packages and, more particularly, to flip chip interconnection to the leadframe.
Leadframe-based packages are used extensively in electronic packaging due to their low cost. The first level interconnection is typically achieved through wire bonding. Recent demands for improved electrical performance, particularly for devices used in RF applications, has led to consideration of xe2x80x9cflip chipxe2x80x9d interconnection to the leadframe.
Conventional flip chip devices employ solder bumps to make contact between the die and the bonding fingers of the leadframe. Devices based on solder bumps have not been accepted for many applications, because of high cost and because the process of soldering bumps to leadframes presents technical challenges, such as solder run-out. What is needed is a package configuration that accomplishes flip chip interconnection to the leadframe but which retains the low cost of conventional leadframe-based packaging.
A flip chip-in-leadframe package includes a chip having bumps that are bonded directly to the bonding fingers of the leadframe. According to the invention, bumps are formed on the die by a stud-bumping technique employing a metal such as gold; and the chip is attached to the leadframe by thermo-compression of the bumps onto the bonding fingers of the leadframe.
Accordingly, in one general aspect the invention features a method for connecting a die to a leadframe, by forming metal bumps on the die, contacting the bumps with bonding fingers of a lead frame, heating the bumps, and pressing the bumps against the bonding fingers. The thermo-compression process results in deformation of the
In some embodiments of the method the bumps are formed by a xe2x80x9cstud-bumpingxe2x80x9d technique employing a wire bonding machine, as is well-known in the chip packaging art. The metal forming the bumps can be any of a variety of metals and alloys having a large range of plasticities. In particular embodiments the metal includes gold.
The bumps can be heated by, for example, applying heat to the die.
In some embodiments of the method, where the die is situated cavity-down, the bonding fingers are supported on a substrate and the back side of the die is supported by a press, and the bumps are pressed against the bonding fingers by applying a force on the press to move the die and the substrate toward one another. The bumps are heated sufficiently to permit plastic deformation of the bump material under the applied pressing force to the extent of 15% to 20% of the original bump height. The combination of pressing force and temperature is selected according to the particular chosen metal or alloy; where gold is used, for example, the bumps are heated to a temperature in the range about 180xc2x0 C.-400xc2x0 C., and a force is applied equivalent to vertical loading in the range about 10 grams to 250 grams per bump. The substrate may, conveniently, have an adhesive surface, onto which the leadframe is affixed, and the substrate may be a sheet or film. In particular embodiments the substrate is a tape having a releasable adhesive, so that the substrate can be removed from the surface once the bumps have been connected to the bonding fingers. This can in some embodiments expose lands on the leadframe for connection.
It may be preferable to provide mechanical support between the die and the substrate. Accordingly, in some embodiments of the method, a measured quantity of fill material is dispensed onto the substrate before the bumps are contacted with the bonding fingers, so that as the die and the substrate are forced to move toward one another the material is compressed between the die and the substrate. Preferred fill materials include adhesive resins. The material is also displaced in the region below the bump as the bump deforms, thereby permitting the bump material to form direct metallurgical contact with the bond finger material. Preferably a sufficient quantity of the fill material is dispensed so that the material is forced to fill the volume defined generally between the die and the substrate. The material may be dispensed by any of a variety of techniques, including for example screen printing and extrusion through a nozzle.
The resulting flip chip-in-leadframe package is highly reliable, as the molding compound functions as the underfill in conventional flip chip packaging, thereby extending the life of the joints formed between the die and the leadframe. Moreover, the process is generally compatible with conventional plastic packaging process flow.
In another general aspect, the invention features a method for forming a plurality of flip chip-in-leadframe packages, by providing a plurality of leadframes each having a set of bonding fingers, providing a plurality of dies each having a set of metal bumps formed thereon, positioning the leadframes onto a support, placing the dies onto the leadframes such that each set of bumps contacts a set of bonding fingers, heating the bumps, and pressing the dies against the leadframes to compress the bumps onto the bonding fingers.
In some embodiments the package is transfer-molded by conventional techniques to form a molded package with any desired footprint such as a leaded or leadless configuration. In some embodiments the chip is situated cavity-up, and in other embodiments it is cavity-down. In some embodiments the back side of the die is exposed to provide improved thermal performance, or for other advantages.
The flip chip-in-leadframe package according to the invention is particularly useful in applications in which the smallest pitch of the bonding pads on the die is at least large enough to match the smallest bond finger pitch achievable in leadframes. The smallest bond finger pitch in currently available leadframes is generally in the order of 150 xcexcm.
In another general aspect the invention features a flip chip-in-leadframe package, made by steps of providing a die having a set of bumps formed thereon, providing a leadframe having bonding fingers, contacting the die with the lead frame so that the set of bumps contacts the set of bonding fingers, heating the bumps, and pressing the bumps against the bonding fingers.
The process according to the invention is faster than conventional wire bonding, and uses considerably less Au (or other costly metal), owing to the fact that only one bond is formed rather than two. Moreover the flip chip attachment does not require use of die attach material and the subsequent curing operation. The flip chip connections are achieved according to the invention using standard wire bondable finish and thereby eliminating the necessity for additional processing or treatment of the leadframe.